Zipponi Thermostatic shower valve leaks between hot and cold

I purchased a new home built in 2007 and the builder is no longer in business. It has a thermostatic shower mixer valve with 3 volume valves above it for the 3 shower heads. It may be a Hansgrohe valve (but I'm not sure). Everything works but I've discovered that the mixer valve has a significant cross-leak between the hot & cold supply.

I've tested this by leaving all 3 shower valves off and turning off the home's main hot supply. With the hot supply off, the other hot faucets in the home will still run at about 50% of the cold faucet rate. Turning the mixer knob changes the "leak rate" (but it doesn't stop it) and I can hear the water flowing inside the mixer valve.

I pulled off the trim and I could post a photo of the valve if needed. It has two 16mm hex caps (left and right) with screw-slot adjustments centered on them.

They serve as both check valves and line shut-offs. I took them apart. The screw is behind the check valve's spring and increases the pressure until it bottoms out and becomes a shut-off. The check valves have o-rings that seat on the casting. Mine don't seat well and the o-rings also don't look good and maybe the springs are weak. I think I need two new check-valve/spring/o-ring combo units.

Huntington Brass told me that many manufacturers use this valve, which is made by Zipponi in Italy. Since my trim design didn't match any of their trims, they wouldn't send me check valves.

I've been searching the Internet and I think my trim design is California Faucets, another Zipponi customer, so I've emailed them. This is turning into a big project.

If anyone knows where to get check valves for a Zipponi, please chime in.

After years of lurking on this forum and finding solutions to various plumbing problems, I'm pleased to post a little "payback" that may help anyone that finds themselves led here by a search.

Two new check valves fully solved my hot-cold cross-leak problem in my Zipponi thermostatic shower valve. I've learned that Zipponi, a manufacturer in Italy www.zipponi.it has supplied this exact thermostatic valve to many manufacturers of shower fixtures worldwide for many years and that calcium or other debris damages the check-valve o-rings and the internal leak worsens (like an external faucet leak does).

After replacing both check valve plungers, springs and o-rings, I reran the test that I described above and there was zero cross-flow. Luckily the metal seats under the o-rings were undamaged. I found a lot of metal pieces stuck in my old o-rings, so before installing the new parts, I flushed the system thoroughly by letting it blast full-force into the shower stall for a good while with both of the hot & cold hex covers and check valves removed. A lot of junk came out.

One tip - the check valves fall apart when removing them, so avoid dropping the parts inside the wall by stuffing a rag in the hole, which also catches any water that would get inside the wall.

Unfortunately, I had no luck locating a source of Zipponi parts and I also never located the manufacturer of my logo-less fixtures (I was wrong about California Faucets).

However, I lucked-out with a friend that had an assortment of miscellaneous parts including two new check valves in unmarked bags that matched my check valves. He had no idea where they came from. They may have been another manufacturer which happened to match the valves used by Zipponi.

I realize this doesnâ€™t help solve your problem. It also doesnâ€™t help me if my valves fail again or anyone else with leaky valves who may be led here by a search engine. My next step was going to be an email to Zipponi, which might get you info on a source and hopefully lead to you adding the final and key element of this thread for the next guy.

Thanks anyways.
After reading your post, I took it apart further. Sure enough, the washers in the built-in check valves have disintegrated.
I will try replacing them since the only Jacuzzi parts list doesn't have that by itself, you have to buy the entire assembly for $370.
If that doesn't work, maybe an external check valve would solve it.